Labour has called on Boris Johnson to clarify who paid for his Caribbean holiday over the New Year.
According to the MPs' register of interests, the accommodation - has a "value" of £15,000 - and was covered by David Ross, the co-founder of Carphone Warehouse.
A spokesman for Mr Ross said the register "is correct" and he "facilitated accommodation" for the PM.
Downing Street said the trip had been properly registered.
The register also shows earnings Mr Johnson received last year before becoming PM, including more than £327,000 for seven speaking engagements, one of which was a three-hour speech where he was paid £122,899.74.
The prime minister took the holiday to St Vincent and the Grenadines with girlfriend Carrie Symonds between Boxing Day 2019 and 5 January 2020.
Labour's shadow minister for the cabinet office, Jon Trickett, said Mr Johnson "must come clean" about the holiday accommodation, adding that if he does not, Parliament's standards watchdog "should step in".
"The public deserves to know who is paying for their prime minister's jaunts," Mr Trickett added.
The entry in Mr Johnson's register of interests says Mr Ross donated accommodation "for a private holiday for my partner and me, value £15,000".
But a spokesman for Mr Ross told the Daily Mail: "Boris wanted some help to find somewhere in Mustique, David called the company who run all the villas and somebody had dropped out.
"So Boris got the use of a villa that was worth £15,000, but David Ross did not pay any monies whatsoever for this."
A later statement from the spokesman added: "Mr Ross facilitated accommodation for Mr Johnson on Mustique valued at £15,000.
"Therefore this is a benefit in kind from Mr Ross to Mr Johnson, and Mr Johnson's declaration to the House of Commons is correct."
Mr Ross has not provided any further details as to what he means, in this context, by a 'benefit in kind.'
But sources in Westminster have suggested to me that this could refer to some sort of swap whereby David Ross agreed to give up his property - at a later date - in order to facilitate the prime minister's stay elsewhere on the island.
And that there was no kind of cash donation.
Mr Ross was one of Mr Johnson's aides in City Hall and was appointed to the Olympics organising committee.
But he resigned from the roles, and his company, over a share scandal in 2008.
It emerged Mr Ross had used millions of pounds' worth of Carphone Warehouse shares as collateral against personal loans without informing the company's other directors - a potential breach of City rules at the time.
Mr Ross has been a long-standing donor to the Conservative Party, pledging £250,000 in the last election campaign.
A Downing Street spokesperson said: "All transparency requirements have been followed, as set out in the Register of Members' Financial Interests".
Mr Johnson faced criticism over his holiday for not returning sooner, after the US killing of Iranian general Qasem Soleimani raised tensions in the Middle East.
It is the first trip abroad that Mr Johnson has declared since going to Saudi Arabia in September 2018.
Only one other MP has declared a free holiday in the last year.
Mr Johnson also declared payments he had received prior to becoming prime minister, including book royalties and hundreds of thousands of pounds for speaking engagements.
In the first six months of 2019, Mr Johnson earned more than £327,000 for the seven speaking engagements, which lasted a total of 17.5 hours.
He was also paid £22,916.66 a month for his column in the Daily Telegraph newspaper, which was published weekly.